Add Theme::TextStyle Format::textStyle() const;

Summary: This is required for using KSyntaxHighlighting in KTextEditor.

Test Plan: make && make test

Reviewers: cullmann, vkrause

Reviewed By: vkrause

Subscribers: kwrite-devel, kde-frameworks-devel

Tags: Kate, Frameworks

Differential Revision: https://phabricator.kde.org/D14471